It is well known that a two-stroke engine has the advantages of a simple structure, lower manufacturing cost, being easy to repair, and a steady power output; however, its drawbacks are the higher consumption of fuel and the higher pollution caused by exhausted waste gas. The aforesaid drawbacks have become rather serious problems under the tendency pressure of saving energy and environment protection.
The aforesaid drawbacks of higher fuel consumption and pollution are mainly caused by part of the pre-mixed fuel gas to be exhausted together with the waste gas during the scavenging stage in the engine; the volume of the pre-mixed fuel gas exhausted generally reaches ranging from 25% to 40%.
Conventionally, the method of preventing the fresh gas from being exhausted during exhausting stroke is to properly design the length of the exhausting pipe, the length of the divergent cone and the convergent cone so as to let the divergent cone and the convergent cone cause the exhausted wate gas to generate negative pressure and a positive pressure respectively to the cylinder. The matching method between the positive and negative pressure and the scavenging and exhausting time is called "exhausting tuning." The preferred relation among the aforesaid stages is that when the piston is nearing the bottom dead point, and during the scavenging ports being opened, the exhausting port should be under the negative pressure state; during the period from the scavenging ports being closed to the moment of the exhausting port being closed, the exhausting port is under positive pressure state. The transmission speed (at sound speed) of the pressure wave has no relation with the RPM of the engine; consequently, the pressure wave of the exhausted gas can, after the size of the exhausting pipe being set, furnish a tuning effect only within a given RPM range the engine. Whenever the engine running at other speeds, there will be no tuning; instead, an adverse effect will be generated; for example, an early pressure wave may hinder the negative pressure, and lower the exchanging ratio of the gas. A late pressure wave may cause the fresh air to leak and to reduce the volumetric efficiency of the cylinder.
In view of the aforesaid problem of the fresh air leaking during the scavenging stage of the two-stroke engine, the inventor has developed a rotary valve to rotate syn-chronously with the crankshaft (either in the same rotating direction or not). The rotary valve has a slot to be aligned with two passage on the round hole seat at a given time so as to have the compressed air in the rotary valve jetted into the exhausting port or the crankcase for the purpases of blocking the fresh air from leading out and increasing the pressure of the fresh air in the cylinder, i.e. increasing the pressure of scavenging air. The exhausting efficiency of a two-stroke engine can be elevated as a result of the aforesaid valve structure.